The present invention relates to noise reduction, and in particular, to circuits and methods to reduce noise in oscillator circuits.
Noise is a common problem in circuit design. Noise in integrated circuits may be caused by a variety of phenomena and may adversely affect the performance of the circuit or system in which the circuit operates. FIG. 1 is an example of a typical oscillator circuit that is affected by noise. Oscillator circuit 100 includes an inductive load “L” and capacitive load “C” that are driving by a pair of switching transistors 101 and 102. In this example, the switching transistors 101 and 102 are configured as a cross-coupled pair, where a gate of transistor 101 is coupled to the drain of transistor 102, the gate of transistor 102 is coupled to the drain of transistor 101, and the sources are coupled together and to ground. Ideally, oscillator circuit would resonate at the following frequency:ω=1/sqrt(LC).However, noise in the circuit will cause the oscillation frequency to vary from the ideal resonant frequency. Such variations from the ideal resonant frequency may cause deleterious effects on the performance of other system functions.
To understand how noise affects a circuit, such as an oscillator circuit, it is helpful to understand how individual devices in the circuit introduce noise into the system. FIG. 1 further illustrates the noise generated by MOS devices, such as the transistors 101 and 102. Plot 150 shows two noise components. The first noise component is referred to as flicker noise (or 1/f noise). As illustrated in plot 150 at 111, flicker noise decreases with increasing frequency. For example, flicker noise may decrease at a rate of −10 db/decade. The second component of noise is thermal noise, which is illustrated at 112. Thermal noise is regarded as the same across frequencies. As illustrated in plot 150, at some frequency fk the flicker noise 111 may fall below the thermal noise 112.
When devices are combined into a circuit, the noise components are converted into system errors or system noise. For example, plot 160 in FIG. 1 illustrates how the device noise components of from plot 150 are converted into phase noise in oscillator 100. As illustrated in plot 160, flicker noise is converted into a −30 dB/decade phase noise component 113 and the thermal noise is converted into a −20 dB/decade phase noise component 114. The intersection of these two noise components occurs at a frequency of Δfk1, which is sometimes referred to as the noise corner frequency. This phase noise profile expresses the frequency component shifts in the oscillator output signal around the resonant (or center) frequency of the oscillator.
From plot 160, it can be seen that low frequency noise components can cause a significant impact on a circuits performance, including degradation of the frequency stability of an oscillator, for example. Thus, it is desirable to reduce noise in oscillators and other circuits. The present invention solves these and other problems by providing noise reduction circuits and methods.